Add support for specifying first and last name.

[glitchedmob]

Updates the controller to accept first_name and last_name parameters.
Updates the service subscribe method to accept an optional $firstName and $lastName parameter.
Removed some extra spaces from various lines (something my editor automatically does when I save).

[daltonrooney]

Hey, thanks so much for submitting this PR! Really glad to see people are getting some value from this plugin.

I've actually got an active branch going with some structural changes to the way submissions are handled, and I think it will conflict with this PR. So I think I'm going merge that first, and then I'll integrate your changes by hand. (I'll also try to watch out for extra whitespace. I should find out why my editor isn't doing that automatically)

Since we're adding support for additional fields, are there any other fields that may be broadly useful? In the future I'm thinking about adding support for P&T's Contact Form plugin, so theoretically we could be passing a whole array of contact fields.

[glitchedmob]

No big deal on not merging right now. I ended up needing this change right away for a client project so I'm just running a forked version of this plugin for now.

I could see support for P&T's Contact Form being pretty useful. Usually I end up having to just make form submissions happen over ajax to support a single form doing multiple things. Such as the contact form also saving user's info to Mailchimp or Constant Contact.

As for adding fields, that's a really good idea. I just took a look at the "Add a new contact" form in the Constant Contact UI and they have a lot more fields that could be used..

I doubt support for all of them would be necessary, but some like phone number, job title and company I could see being very helpful.